Synchronous detection is a signal processing method used to extract weak signals from a noisy background. This method may be implemented using conventional lock-in amplifiers.
Synchronous detection requires that the signal of interest be modulated at a very stable frequency and the detector, at a receiving end, have access to the modulation signal (reference signal). The detected signal and the reference signal must also be in phase with each other.
FIG. 1 illustrates a conventional synchronous detection device 10. The signal of interest is modulated by a reference signal at a frequency ωm (typically tens of Hz to a few kHz). The signal of interest and the reference signal are both multiplied, or mixed together, and the output signal is fed to a low-pass filter. As shown, the signal of interest, As, is provided to modulator 12 and then modulated by a reference signal, sin (ωmt), generated by reference source 18. The modulated signal, As sin (ωmt), is multiplied with the reference signal in mixer 14 and then low-pass filtered by low-pass filter 16.
Mathematically, the mixing process may be described as multiplication of the signal of interest given by:Ys(t)=As sin(ωmt)  (1)with the reference signal expressed as:Yref(t)=sin(ωmt)  (2)to yield an output of:Yout(t)=(As/2)(1+cos(2(ωmt))  (3).
Subsequent low-pass filtering rejects the component at twice the modulation frequency and yields the amplitude of the signal of interest (As), which is the quantity to be measured.
FIGS. 2A-2C show schematic representation of synchronous detection in the frequency domain. The mixing process has the effect of down-converting the signal of interest from a frequency above the modulation frequency to DC (ignoring the high frequency component). The information of interest, which is the amplitude of the signal, is preserved. Because the output frequency of the mixer is near DC, it may be passed through a filter with a small bandwidth. Narrowing the bandwidth of this filter allows noise to be reduced without reducing the signal. This results in high signal-to-noise ratio (SNR) values, enabling weak signals to be extracted from noisy backgrounds.
Theoretically, lock-in amplifiers assume that the signal amplitude is a DC quantity, thereby allowing the low-pass filter to have a very small cut-off frequency. In actuality, the signal may be quasi-DC in the sense that the amplitude may be slowly varying as a function of time. Accordingly, the low-pass filter cut-off frequency must be large enough to accommodate the rate of variation of the signal amplitude As without loss of information, as shown in FIG. 2C. For example, the motion of a platform may cause the information content to vary at a rate typically in excess of a few tens of Hz.
Synchronous detection requires that the signal of interest and the reference signal both be in phase with one another. In a laboratory environment, this may easily be achieved. In a remote sensing environment involving active optical sources, however, the return time of the optical signal varies as a result of changes in the distance between the transmitter and the target. This results in a variable phase between the signal of interest and the reference signal. Limiting this variable phase introduces additional levels of complexity to synchronous detection devices.
Synchronous detection, or lock-in methods are advantageous over other detection methods, because they reduce noise components near the signal of interest. The mixing process shifts the frequency of the signal of interest from the modulation frequency down to DC; it also shifts the noise components near DC that are present at the input of the mixer up to the modulation frequency. These components, however, are rejected in the low-pass filter. The result is a reduction in 1/f noise present in the signal at the input to a synchronous detector.
Another advantage is that synchronous detection only yields information on the modulated portion of the signal provided at the input of the synchronous detector. This is particularly useful for active sensors, because modulating the source (laser) instead of chopping an input to the receiver allows photo-currents associated with interactions of a target and the source to be separated from background noise. Additionally, the method is also superior to simply modulating the source and using a band-pass filter at the modulation frequency to isolate the signal of interest from the background noise. It is generally much more difficult to realize a very narrow filter about a center frequency of tens of Hz to a few kHz than it is to build a filter near DC.
Synchronous detection for a single pixel detector has been developed. Synchronous detection for arrays of pixels in a FPA, however, has not been developed, most likely due to the difficulty in providing an independent synchronous detector for each pixel in the array. A FPA having 256×256 pixels requires 65,536 independent synchronous detectors.
Conventional lock-in amplifiers with synchronous detection have been developed based on digital filtering technology. These are not used in FPAs, however, because of impractical digital processing rates needed to process the data from all the pixels in the FPA. For example, the FPA would need to be operated at a frame readout rate of several times the chopping frequency (modulation frequency), and the data then digitized and processed for each pixel. This would require that the FPA be operated at several thousand frames per second (including digital conversion) and that the processing throughput keep up with this data rate.
While, conceptually, FPAs with large number of taps and analog-to-digital converters (ADCs) of up to one per column may be implemented, this approach has several drawbacks. First, operation at several thousand frames per second for an array with 256 or more pixels per row requires that each tap and ADC operate at several million pixels per second. The high bandwidth required for this would result in a substantial read-out noise penalty. Additionally, the ADC power consumption would be high, if an ADC, with high number of bits (typically 14+) and high readout rates (>1 MSPS), is needed for each column. As a practical matter, there would also be a significant risk of cross-talk and noise associated with capacitive coupling due to the large number of high-speed digital signals present. Finally, real-time digital signal processing needed to implement synchronous detection would be a major challenge in such a high data-rate environment (e.g. 256×256×5,000 FPS=327 million words/sec).
Accordingly, a need exists to provide a FPA with synchronous detection capability. No practical solution has thus far been suggested. This invention addresses this need and discloses several solutions.